Mobile communication is increasingly popular and, more and more, cellular service providers are focusing on techniques for high-capacity and high-quality communication of information over wireless links.
In 1998 the Chinese Wireless Telecommunications Standards proposed to the International Communications Union a new standard that is based on Time Division Duplexing (TDD) and Synchronous Code Division Multiple Access (CDMA) technology (TD-SCDMA) for TDD. The International Communications Union has approved and adopted this proposal. In a TD-SCDMA system, time slots and spreading codes separate the users in a cell. The adopted system has several advantages over 2nd generation and other 3rd generation communication systems.
Cells are distinct geographic areas serviced by a wireless telecommunications system, and, depending on the topography of the terrain surrounding the cells, they may have irregular shapes. Typically, each cell contains a base station that communicates with the wireless terminals in that cell and with the Wireless Switching Center, which is the heart of a typical wireless telecommunications system. The Wireless Switching Center is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless and a wireline terminal.
Often, the signal transmitted by a wireless terminal to a base station is radiated omnidirectionally from the wireless terminal. While some of the transmitted signal may reach the base station in a direct, line-of-sight path, most of the transmitted signal radiates in other directions and never reaches the base station. However, some of the signals that radiate initially in a direction other than towards the base station strike an object, such as a building, and are reflected towards the base station.
Therefore, a signal can radiate from the wireless terminal and be received by the base station via multiple signal paths. Such a signal and its reflections arrive at the base station at different times, after having traveled on different paths, and will interfere to form a composite of several constituent signals. This is known as “multipath” interference. Furthermore, the characteristics of each received signal are affected by the length of the path traveled and the objects the signal has been reflected from.
In a CDMA system each radio receiver attempts to identify and isolate the highest-quality constituent signals of a composite multipath signal and to demodulate and recombine them to form an estimate of the transmitted signal. This process is conducted with, among other things, a RAKE receiver. A RAKE receiver uses several baseband correlators and individually processes multipath signal components, attempting to identify the strongest constituent signals in the composite signal. Each correlator in a RAKE receiver is called a “finger.” The RAKE receiver then isolates and demodulates each of the strongest constituent signals, and then recombines them to produce a better estimate of the transmitted signal than could be obtained from any single constituent signal
Because each received signal travels a different path, any discrepancy is manifested as a relative time delay, or phase shift, in the constituent signals. Any phase shift in a constituent signal that does not exactly equal an integral number of wavelengths of the carrier signal translates into a partial phase rotation in the constituent signal with respect to the other constituent signals. The partial phase rotation of the constituent signals at the receiver is irrelevant and does not affect the demodulation process if the modulation scheme of the transmitted signal does not function by modulating the phase of the carrier. In contrast, the partial phase rotation of the respective signals must be considered in the demodulation process if the modulation scheme of the transmitted signal functions, at least in part, by modulating the phase of the carrier signal (e.g., quadrature phase-shift keying, quadrature-amplitude modulation, etc.). Typically, the partial phase rotation of the respective signals is accounted for by realigning their phase.
In the prior art, a technique called “pilot-aided CDMA” facilitates the task of realigning the phase of the respective constituent signals. In a pilot-aided CDMA system a pilot signal is transmitted in the same channel as the information-bearing signal and traverses each path from the transmitter to the receiver, and is subject to the same environmental factors as the information-bearing signal. Because the RAKE receiver knows that the phase of the pilot signal, as transmitted, is invariant, it can estimate the phase rotation of each constituent information-bearing signal by comparing that signal to the pilot signal and its reflections.
In many environments such as crowded cities, fading, which is related to multipath interference, can become quite severe. The term “fading” is used when the amplitude of the received signal drastically varies as a result of the phase difference between a signal and its reflections. Such signals, at times, can weaken or practically cancel each other, or can combine to form a stronger signal. In a wideband direct-sequence spread-spectrum CDMA communication system (WCDMA), where signals use separate slices of the total available wideband, different multipath components fade independently and the diversity reception of the signal is the method of choice. Most often the RAKE receivers are used to implement the diversity reception technology. The RAKE receiver, in turn, is based on the path search method. In general, no matter which kind of diversity reception technology is adopted, path researching is required. However, the conventional path search structures entail a large storage space to implement the necessary correlation algorithms.